Wind power refers to the conversion of wind energy into more useful forms of energy, such as electricity. Wind energy is an attractive alternative to fossil fuels because it is plentiful, renewable, widely distributed, clean, and produces no greenhouse gas emissions. Wind energy currently accounts for about 1.5% of worldwide electricity usage, and approximately eighty countries around the world use wind power on a commercial basis (World Wind Energy Report 2008: Report, World Wind Energy Association, February 2009; and Worldwatch Institute: Wind Power Increase in 2008 Exceeds 10 year Average Growth Rate, May 2009 ). Further, world wind generation capacity has more than quadrupled between the years 2000 and 2006, doubling about every three years.
Offshore wind turbines harness the energy of powerful winds native to deep sea waters to provide electricity. Although necessary for energy production, these same strong winds result in asymmetrical loads that act on the rotor blades of the turbine, in what are known as bending moments. These high loads are subsequently transferred to the turbine shaft and ultimately, to the gearbox of the turbine, which results in gearbox failure and the unavailability of the turbine. Attempts to mitigate the high structural loads associated with bending moments include attaching the rotor blades to a flexible structure with limited pivoting capability, known as teetering hinge.
Conventional teetering hinges are based on mechanical devices that use bushings or ball bearings. Due to the rigid nature of these devices, conventional teetering hinges lack substantial capability to absorb sharp dynamic loads. Furthermore, the continued exposure to high loads combined with the limited angling ability of the hinge results in the degradation of the metallic bearings by pitting. Also, the need to center the rotor blade axis back to a point perpendicular to the shaft axis requires complicated centering devices based on metallic or elastomeric springs outside the bearing itself.
Other conventional teetering hinges are based on single metal-elastomeric bearings. In these devices, the preload cannot be controlled or adjusted. Rather, the preload is obtained by permanently transferring loads to the hub and the shaft, which causes unnecessary and potentially dangerous stress over the life of the system Like the conventional teeter hinges based on mechanical devices, these hinges also provide less than optimal reliability.
Accordingly, there is a need for a teetering hinge suitable for two-bladed wind turbines with improved reliability, better durability, and the enhanced ability to handle high structural loads.